If you have a few plugins in different folders and you want Firefox to detect them, set MOZ_PLUGIN_PATH enviroment variable to the list of folders you want Firefox to look in.

Yeah, I searched here and there looking for exactly this and just now found it. Useful, because I installed Quicktime Alternative which has a configuration program that modifies the dlls, I think, and it does not change the configurations if I move or copy the dlls to the Firefox plugins folder.